A Nasa scientist has proposed using the replacement to the space shuttle to visit a near-Earth asteroid.

The Crew Exploration Vehicle (CEV) is due to make its maiden flight in 2014, with the eventual aim of ferrying astronauts to and from the Moon.

Dr Paul Abell said such a mission could help efforts to protect against an asteroid on course to hit Earth.

Currently, the project is envisaged to include two or three crew members and last a total of 90-180 days.

This would be the first time humans left the Earth-Moon system

Paul Abell

Dr Abell presented an outline of his mission concept here at the Lunar and Planetary Science conference in Houston, Texas.

The plan would be to visit an asteroid in the sub-kilometre size range, perhaps about the size of asteroid Itokawa (535m) which was visited by Japan's Hayabusa spacecraft.

'No showstoppers'

The CEV, which Nasa has named Orion, would spend approximately 7-14 days at the asteroid.

The spacecraft would effectively orbit the body, while astronauts approach the surface to carry out experiments.

The mission could involve robots, to take on any task deemed too dangerous for an astronaut.

Japan's Hayabusa captured its shadow on the surface of Itokawa

"We carried out this feasibility study to see if there were any showstoppers.

"The answer seems to be 'yes, this can be done'," Dr Abell told the BBC News website.

He added: "It's a very exciting mission. This would be the first time humans left the Earth-Moon system."

At the moment, the mission is still very much a concept. The team will wait for the results of a Nasa evaluation of the feasibility study before proceeding any further with the idea.

Planetary defence

Characterising the internal structure of a near-Earth asteroid could help scientists develop better ways to divert or destroy an asteroid that was headed on a collision course with our planet.

It could also carry out valuable science on the surface, including studies of the asteroid's orbital motion and spin rate. Returning samples to Earth would provide insights into the composition of the Solar System 4.5 billion years ago.

In addition, Dr Abell said, the mission could train astronauts to "live off the land" on long haul missions, such as a trip to Mars. This might include extracting hydrogen and oxygen from rocks to make water.

Orion will ride atop a single five-segment solid rocket booster

A mission would probably involve sending several robotic satellites as "advance parties" to potential targets.

They would determine which asteroids were safe destinations for humans. Factors preventing manned exploration might include a jagged surface, a rapid spin rate, and debris surrounding the asteroid; those which turned out to be binary objects, where two bodies orbit each other, could also be excluded.

A mission would be expected to take longer than a journey to the Moon, but would require less energy.

The space shuttle is due to be retired in 2010 to make way for the CEV.